In recent years, various wireless communication technologies, such as Bluetooth, Wireless Local Area Network (WLAN), and Femtocell technologies, are widely used. Wireless communication by these technologies is free from the trouble of connecting communication terminals with a cable. These technologies are also convenient for users since it is easy to set wireless connection.
However, each of these wireless communication technologies has a problem to be solved. For example, in the Bluetooth, access points are not used for communication between terminals, but a valid communication distance between terminals is short. Accordingly, its application is limited to, for example, transmission of audio data from audio instruments to earphones. The WLAN is a technology that provides terminal-to-terminal communication via a relay device called an access point. Since a wireless communication network is formed by using access points, it is difficult for users to make a wireless connection outside the wireless communication network. The Femtocell is an ultra-small base station system which becomes popular in recent years. Since wireless communication is performed via a base station, the femtocell communication is limited to the service area of each base station. Under these circumstances, Device-to-Device (D2D) communication is attracting attention as a new wireless communication technology.
The D2D communication is a wireless communication technology in which portable devices, such as cellular phones, directly communicate with each other without using a base station. The D2D communication has an advantage of being able to secure a long valid communication distance as compared with other wireless communication technologies which provide communication without using the base station. If the D2D communication is used, data communication can be performed at a maximum rate by making full advantage of the capacity of each terminal. Since users directly communicate without using the base station in the D2D communication, communication delay between terminals is advantageously smaller. The D2D communication further has an advantage that communication congestion in each base station can be eased as the number of users increases.
The D2D communication is expected to be used for various applications. For example, the D2D communication is expected to provide users, who are in the service range of the D2D communication, with service of the Social Networking System (SNS), and/or is expected to be used for advertisement distribution. The D2D communication is further expected to be used as a communication method at the time of disasters in which the base stations are not available.
With regard to the conventional wireless communication technologies, see Japanese Laid-Open Patent Publication No. 2008-017318 and Japanese Laid-Open Patent Publication No. 2008-245242, for example.
However, the D2D communication has a problem in that an optimal frequency is not selected therefor.
Since the frequency for use in the D2D communication is not appropriately set in the conventional technologies, some of the throughputs in the D2D communication are lower than an expected throughput. Communication interruption may also occur during some of the D2D communication sessions.